The panel of a digital X-ray detector is scanned row by row. Each row includes pixels that are read out at the same time. Any time-varying interference that affects the pixels is row correlated noise (RCN) in an image generated by the X-ray detector. The time-varying interference can result from electromagnetic interference (EMI), mechanical vibration, and/or power supply noise.
More specifically, RCN is an offset-type of noise that is independent of X-ray signal. RCN can be directly calculated from offset or dark images. RCN can be calculated by generating a row average vector of the offset difference image that is created by subtracting two sequential offset images and then calculating the standard deviation of the vector. Since the visibility of image artifact is related to the background noise, an RCN index is defined as the ratio of RCN artifact to the standard deviation of the difference image. The RCN is a measured, empirical quantification of the RCN of a digital image detector at a particular point in time. The RCN index can be calculated as follows:
                    RCN_index        =                                            σ              RCN                                      σ              background                                -                      1                          number_columns                                                          Equation        ⁢                                  ⁢        1            
where σRCN indicates RCN artifacts while the σbackground is the standard deviation of the whole offset difference image representing the background noise. The term 1/sqrt (number_columns) is further subtracted to yield an unbiased estimate of the RCN index. In addition to the whole image, the RCN index can also be calculated based on region of interest (ROI) on the image. To have robust estimate or to capture potential and maximum value of the RCN Index, a sequence of offsets can also be used. For example, in digital mammography and RAD systems, an RCN test has been introduced during system boot or in system diagnosis or in a routinely system quality check.
The RCN artifact is easy to see in low dose X-ray images. With the increasing X-ray dose, the impact of RCN gradually decreases until the ratio is below the visible threshold. However, the current RCN index value is calculated based on offset images without X-ray and compared to a constant threshold.
There are at least two conventional methods to ameliorate the effects of RCN: One conventional method of ameliorating RCN in a digital image detector is to design the detector that is less susceptible to the interference. For instance, the digital image detector can be designed with a greater EMI shield to protect from EMI and more rugged parts to protect from mechanical vibration. The measures, however, will increase weight of the digital image detector which is counterproductive to the increasing customer interest in lighter digital image detectors. These measures to ameliorate RCN also increase attenuation of the X-ray radiation to the extent that more X-ray radiation is required to achieve the same image quality, which is counterproductive to patient safety and counterproductive to the increasing customer interest in lower doses of X-ray radiation.
Another conventional method of reducing the effect of RCN is to avoid physical placement of the X-ray system in an environment with high EMI. One way to avoid physical placement of the X-ray system in a high EMI environment is to provide an environmental specification to the customer during system installation that would prevent placement of the X-ray system in a location that has high amounts of EMI. Another way to avoid physical placement of the X-ray system in a high EMI environment requires installation in a room that is specially equipped to shield EMI. However, more and more X-ray systems are designed for mobile environments; most locations for mobile digital X-ray systems are anywhere in the hospital in locations outside of specially equipped rooms. In that case, reducing the effect of RCN is particularly important to ensure image quality.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to reduce the effects of RCN without adding shielding, increasing component strength or requiring placement of the X-ray system in specially equipped rooms.